1. Field of the Invention
The present invention generally relates to processes for forming the end of a tube in which internal surface features are present. More particularly, this invention relates to a process by which the end of such a tube can be reduced and the surface features removed during an end-forming operation while achieving accurate control of the tube outer diameter.
2. Description of the Related Art
Tubes for cooling equipment are often formed to have internal surface features in the form enhancements, which may be described as flutes, ribs, etc., that are present on the internal circumference of the tube to promote heat transfer. Such a tube 10 is represented in FIGS. 3 and 4, in which the latter is a cross-sectional view of the tube 10 showing internal flutes 12 projecting radially inward from the internal circumference 14 of the tube 10. Internal enhancements of the type shown in FIGS. 3 and 4, are typically formed during drawing of the tube 10, and are therefore present along the entire tube length.
In the situation depicted in FIG. 3 in which the tube end 16 is required to undergo an end-forming operation to reduce its diameter, the flutes 12 can interfere with the reduction process and excessively restrict flow through the tube end 16. Therefore, flutes 12 and other internal enhancements are often removed, such as by machining. However, the additional machining step and resulting possible contamination from chips are undesirable. An alternative approach is to remove the flutes 12 during the end-forming operation by using a die to reduce the outer diameter (OD) of the tube, after which the OD die is removed and an inner diameter (ID) mandrel is passed through the reduced portion of the tube. A disadvantage with this approach is that the mandrel can alter the OD of the tube 10, making it difficult to obtain or control the OD of the tube 10.